1. Field of the Invention
The present invention relates to a method of increasing the pH of an acid solution without requiring the use of externally applied electrical energy.
2. Description of the Prior Art
In recent years increased attention has been directed toward the serious problems which result from acid mine drainage. Water which accumulates in both abandoned and currently active strip mines and deep mines tends to be highly acidic, having a pH on the order of about 2 to 3. It also tends to contain a substantial quantity of dissolved salts, such as iron sulfates, for example. Most of the dissolved iron exists in the ferrous state.
Numerous means of dealing with acid mine drainage have been considered. It has been known, for example, to place these materials in confined environments where they will be isolated from environmental weathering in order to minimize acid mine drainage. Such an approach is difficult and not adapted to be effective and economical in the handling of large volumes of materials.
It has also been known to attempt to neutralize the acid solution by using low cost materials such as limestone or lime including such action combined with the concurrent aeration to oxidize the ferrous iron to the ferric state. Air oxidation is relatively slow and is highly dependent upon the pH of the water and the efficiency of the aeration system. Significant oxidation rates do not occur until the solution becomes neutral or basic, i.e., a pH of greater than about 7. As a result, acid mine drainage which has a high ferrous content is typically neutralized in the more expensive lime rather than limestone which is generally considered to be too weak a base to produce a pH above about 5.5.
It has also been suggested to eliminate the formation of acid mine drainage in the overburden by bacterial activity or iron complexation reactions. These procedures interrupt the acid producing chain reaction. This approach is effective on the spoil pile where acid is being produced, not on the acid stream.
It has also been known to suggest electroprecipitation employing a consumable metal anode and an inert cathode with oxidation of the anode being matched by reduction of the dissolved hydrogen ion to hydrogen gas thereby resulting in a pH increase in solution. Heavy metal removal is accomplished in this system by either precipitation of hydrous oxides or by absorption. This system requires an externally applied source of energy.
In the work performed by Tyco Laboratories, Inc. of Waltham, Massachusetts for the Environmental Protection Agency, bearing a publication date of February, 1972, and entitled "Electrochemical Treatment of Acid Mine Waters," various approaches to treatment of mine acid were considered. The use of a rotating disk electrode and electrodes composed of various materials such as carbon, platinum, and stainless steel were discussed. Also disclosed are anodic oxidation of Fe.sup.2+ to Fe.sup.3+ as well as the generation of hydrogen from hydrogen ions taking place at an inert cathode. Use of fluidized bed, packed bed and annular flow reactors are also disclosed. These systems require external energy input.
In spite of the foregoing disclosures, there remains a very real and substantial need for an improved means for neutralizing acid with particular emphasis on acid mine waste.